1. Field of the Invention
The invention concerns the transmission of timing advance data to mobile stations in a cellular mobile radio network such as a GSM (Groupe Special Mobile) network when mobile stations move from one cell to another. The mobile stations are car phones, for example, and the timing advance data must enable a mobile station moving from one cell to another to advance the timing of its transmission of digital data so that it is synchronized with the transceiver station of the new cell. The standard term for this procedure in the GSM system is "handover" and this term is used hereinafter.
2. Description of the Related Art
The following description uses the standard GSM terminology. For more information on this reference may usefully be had to the proceedings of the "Digital Cellular Mobile Communication Seminar" held in Nice from 16 through 18 Oct. 1990.
FIG. 1 shows the structure of a cellular mobile radio network such as a GSM type network.
A mobile station MS such as a car phone, for example, moves inside a cell C1 defined by the geographical coverage of a base transceiver station BTS1. Other cells C2, C3 each comprise a respective base transceiver station BTS2, BTS3. Each of the stations BTS1 through BTS3 is one component of the GSM network and comprises one or more transceivers each associated with an antenna and processing equipment. The cells overlap in part so that there are no shadow areas. The stations BTS1 through BTS3 are managed by a base station controller BSC. The functions of the BSC include management of BTS frequency channels. A BSC associated with a number of BTS constitutes a base station system BSS. Other controllers may also be provided, each controlling a predetermined number of BTS and each being connected to a mobile services switching center MSC which is the master structure of a GSM network. A given MSC can therefore control the operation of several BSS constituting a public land mobile network (PLMN).
A network of this kind operates as follows: the mobile station MS sends streams of digital data in the form of packets to the base transceiver station BTS1 while it is in the cell C1 and the station BTS1 forwards these streams to the BSC which sends them to their destination via the MSC. This destination may be another mobile station or a fixed station.
Each data packet contains speech data, for example, and is transmitted in a 577 .mu.s time slot, eight consecutive time slots constituting a frame. Eight mobile stations can therefore communicate on the same radio channel, i.e. using the same carrier frequency, using time-division multiple access (TDMA). Between two and four channels are usually assigned to each BTS and between 16 and 32 radio channels are therefore available for transmission (and reception) in each cell.
A problem arising in the GSM system is that of synchronizing time slots assigned to a mobile station to the master clock at the BTS. It is necessary to allow for the propagation time of radio waves between a mobile station and its BTS because the mobile stations and the BTS serving them each have their own internal bit clock. As the duration of a time slot is 577 .mu.s and as a radio wave travels 300 m in 1 .mu.s (3.times.10.sup.8 m/s), the mobile station clock must allow for a time shift of 1 .mu.s per 300 m of distance between it and the BTS in order to avoid sending data during the time slot assigned to another mobile station.
FIG. 2 is a correlative timing diagram showing signals sent by the base transceiver station BTS1 and by the mobile station MS. It shows how the appropriate timing advance is communicated to the mobile station.
The base transceiver station BTS1 managing the cell in which the mobile station MS is located sends a clock signal H.sub.0, H.sub.1, H.sub.2, H.sub.4 regularly, at times T.sub.0, T.sub.1, T.sub.2, T.sub.3 and T.sub.4 on a synchronization channel SCH which is part of a broadcast control channel BCCH for transmitting synchronization information to the mobile stations. This clock signal is used when the mobile station must be logged onto a cell of the GSM network, for example when it is switched on or in the event of handover (see below).
The mobile station connects to the network for the first time after it is switched on and can receive the clock signal only from the time MS.sub.ON at which it is switched on.
Given that the mobile station is not usually located at the base transceiver station BTS1, the first clock signal H.sub.1 that it receives after time MS.sub.ON is shifted by a time T relative to the time T.sub.1 at which it is sent by the station BTS1. The signal H.sub.1 is therefore received by the mobile station at time T.sub.1 +T.
At this time the mobile station requiring to connect to the base transceiver station BTS1 sends to the latter on a signalling channel a random access message (access burst). In the case of handover this message is called the handover access message (HA). The duration of each handover access is less than that of a burst constituting a normal signal (normal burst), one containing speech data, for example, and accordingly cannot interfere with signals sent by another mobile station in another time slot.
On receiving this signal (at time T.sub.1 +TA) the base transceiver station BTS1 can determine the timing advance TA between reception of this signal and transmission of the clock signal H.sub.1. This is equal to twice the time to transmit a signal between the mobile station and the base transceiver station BTS1, i.e. twice the time T. The base transceiver station BTS1 then sends a message to the mobile station over the access grant channel AGCH to tell it that it must send its signals with a timing advance TA relative to its clock signal: the mobile station can then send normal signals without risk of them overlapping with those sent by other mobile stations. This ensures that the signals sent by the various mobile stations on a given transmission channel arrive in succession at the base transceiver station BTS1.
This ensures that the signals sent by the various mobile stations on a given transmission channel arrive in succession at the same BTS without these signals overlapping. However, it is necessary to synchronize the mobile stations frequently because their distance from the BTS serving them may vary.
The problem of managing movement of the mobile station from one cell to another is well known. In FIG. 1 the mobile station receives signals not only from base transceiver station BTS1 but also from base transceiver stations BTS2 and BTS3. If the power of the signals received from base transceiver station BTS1 falls below that of signals received from base transceiver station BTS2, for example, the BSC connects the mobile station to base transceiver station BTS2 from which transmission then continues. This is the typical situation when the mobile station moves away from base transceiver station BTS1 and towards base transceiver station BTS2. It is then necessary to modify the timing advance TA so that the mobile station is synchronized to the base transceiver station BTS2 of the new cell C2.
There are three known types of handover for achieving such synchronization: synchronous handover, pseudosynchronous handover and asynchronous handover. Which type is used depends on whether the base stations are respectively synchronized, have an internal clock at the same frequency and of known phase, or have asynchronous clocks whose relative phase is unknown.
Synchronous handover consists in controlling the clocks of the various BTS of a given GSM system so that their clock signals are synchronized. It is therefore unnecessary to supply a mobile station with a new timing advance when it moves from one cell to another because the new timing advance is deduced immediately from that previously used. To be generally adopted this solution would require synchronization of all BTS, however, and would therefore be costly to implement.
To alleviate this problem pseudosynchronous handover is used to synchronize a mobile station to the clock of the BTS of the new cell allowing for the time shifts between the clocks of the old and new BTS. This type of handover is described in European patent application n.degree. 0 398 773 in the name of MATRA COMMUNICATION published on 22 Nov. 1990, for example. This solution has the drawback that it is complex to implement and that the BSS requires a learning phase.
Asynchronous handover is the simplest method to implement and therefore the method most widely used. FIG. 3 shows the general principle. Consider the case where the mobile station MS leaves cell C1 to enter cell C2. Eight successive transmission steps are necessary.
Step 1 is that in which the mobile station MS sends the base transceiver station BTS1 a message MEAS REP equivalent to a cell change request. This standardized message is sent every 0.5 s. In step 2 the base transceiver station BTS1 transmits this information (message MEAS RES) to the BSC which decides if handover is needed. This decision is taken if the message MEAS REP indicates that the mobile station MS is no longer receiving under optimum conditions and that the mobile station is receiving the signals sent periodically by the station BTS2 better than those sent by the station BTS1, for example. The BSC and the MSC can also decide for themselves if handover is needed, allowing for other criteria in taking their decision. In step 3 the base station controller BSC activates a channel of the base transceiver station BTS2 (message CHAN ACT) and the latter acknowledges the allocation (message CHAN ACT ACK). In step 4 the base station controller BSC sends a handover command (message HANDOVER CMD) to the base transceiver station BTS1 which immediately forwards it to the mobile station MS transparently. The handover procedure at the mobile station MS then begins (step 5). The mobile station sends successive messages HA (handover access) to base transceiver station BTS2 with a null timing advance, i.e. without allowing for the distance between them. This step is that described previously with reference to FIG. 2. The new timing advance that the mobile station MS must use is unknown to it and the base transceiver station BTS2 has to calculate it and supply it (message PHYS INFO including the TA). The station BTS2 also sends a message HO DETECTION (TA) to the BSC. In step 6 the mobile station MS sends a connection message SABM to the base transceiver station BTS2 using the new synchronization. The base transceiver station BTS2 advises the base station controller BSC of this (message ESTABLISH INDICATION) and signals to the mobile station MS that it has understood correctly (message UA). In step 7 the mobile station MS sends a message HANDOVER COMPLT to the base transceiver station BTS2 to advise it that the handover procedure is finished. This station immediately forwards this message to the BSC. The BSC then advises the mobile services switching center MSC (message HANDOVER PERFORMED). In step 8 the BSC sends the message RF CHAN REL to the base transceiver station BTS1 to tell it to release the time slot previously allocated to the mobile station MS. The base transceiver station BTS1 responds with the message RF CHAN REL ACK. At this stage the mobile station MS is communicating with the base transceiver station BTS2 which has allocated it a time slot in a frame carried by a given carrier and a timing advance TA.
The main drawback of asynchronous handover is that the sending of the timing advance TA to the mobile station is time-consuming: it takes about 40 ms plus about 10 ms for calculating the TA. The mobile station is not able to send data during this time. Also, sending of the signals HA is mandatory in the GSM system and takes 5 ms per signal. Other delays contribute to delaying handover and calls are frequently interrupted for more than 100 ms. Speech extrapolation software is available for masking interruptions in transmission but is effective only for a subjectively determined time dependent on the hearing of the persons concerned. It is generally agreed that an interruption in communication exceeding 80 ms is audible to a person with good hearing whereas a party paying no particular attention to the quality of the speech signals received may not notice for 200 ms that what he is hearing is not what he should be hearing (because the signals are generated by the speech extrapolation software).
Likewise the mobile station user cannot receive speech type data while the mobile station is in the handover phase. This drawback is a particular problem in urban areas where cell sizes are smaller and the user may pass through several cells during the course of the same conversation. A system of this kind is not comfortable to use for this reason.